selftest: Always set PYTHON to something sensible (if it isn't set)

selftest: Always set PYTHON to something sensible (if it isn't set)

Lots of test scripts need to run with the correct version
of python. With the correct shebang the script should run with the
correct version, the problem is that not all scripts are part
of the installation, some scripts are part of the source code,
and the shebang is not dynamically generated as yet.
It is safer if we are somewhat version neutral at the moment and
ignore the shebang and always run scripts from the test environment
with the python version (determined by PYTHON env variable) If this
env variable isn't set then set it according to the python version
that is running the tests

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>